## Single Phase unexcited Synchronous Motor

• The characteristic of single phase unexcited synchronous motor in which excitation does not necessary is given below.
( 1 ) It operates on single phase supply.
( 2 ) The stator rotating magnetic field rotates at synchronous speed.
( 3 ) It does not require DC Excitation.
( 4 ) It is self starting.

### Principle

• When a magnetic material is placed in the magnetic material, it always aligns in the minimum reluctance path.

### Stator

• The stator of the reluctance motor is similar to that stator of the single phase induction motor
• It consists of starting and running winding in the stator slots.
• This type of motor is also called as Split phase reluctance motor.

### Rotor

• The rotor of the reluctance motor is of salient or projecting poles.
• Let us consider that the rotor of the squirrel cage induction motor consists of 24 copper bars.
• If the rotor bar 5, 6, 11, 12,17,18,23 and 24 are cut, it is similar to 4 salient poles.

### Working

• When a single phase supply is given to the stator winding, a rotating magnetic field is produced in the stator winding.
• When a salient poles rotor cut this magnetic field, rotor aligns in the minimum reluctance path due to reluctance torque.
• The reluctance depends upon air – gap between stator and rotor.
• Figure A shows 4 pole salient pole rotor in which direction of four high Permeance and four low Permeance is shown.
• High Permeance means higher magnetic conductivity and higher inductance. Similarly low Permeance means lower magnetic conductivity and lower inductance.
• The reluctance is inverse of Permeance. Low reluctance means higher inductance and vice versa.
L α N2 / S
Where L = Inductance and
S = Reluctance of magnetic path
• Low air – gap means low reluctance and vice versa
S = L / μ0μra
Where L = Length of air – gap
μ0 = Absolute permeability = 4π × 10 – 7 Henry / meter
μr  = Absolute permeability
a  = Area
• There is low reluctance path between stator and salient poles due to small air – gap whereas high reluctance path between stator and inter – polar axis due to large air – gap.
• The reluctance motor starts as an induction motor.
• When the rotor rotates at its maximum speed, it aligns with the stator synchronous magnetic field due to reluctance torque.
• The angle between stator poles and rotor poles of opposite polarity is called as torque angle.
• As the torque angle increases, the reluctance torque also increases.
• The maximum reluctance torque attains at torque angle of 450
• The load taken by the reluctance motor is only fraction of the load taken by the three phase inductance motor.

• Low maintenance
• DC supply not necessary
• Simple construction
• Constant speed characteristic

• Low efficiency
• Low power factor
• Only fraction of load taken as compared to three phase induction motor

### Applications

• Automatic regulator
• Signaling devices
• Recording instruments
• Tele printer
• Timer circuits
• Gramophone
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